1. Field of the Invention
The present invention relates to methods for controlling isolated bidirectional power converters. More particularly, the present invention relates to methods for controlling dual-active-bridge (DAB) bidirectional converters.
2. Description of the Related Art
Bidirectional converters are increasingly being used in power systems with energy-storage capabilities, such as “smart-grid” and automotive applications. Generally, bidirectional converters are often used to condition charging and discharging of energy-storage devices, such as batteries and super-capacitors. For example, in automotive applications, isolated bidirectional dc-to-dc converters are used in electric vehicles (EVs) to provide bidirectional energy exchange between the high-voltage (HV) battery and the low-voltage (LV) batteries, while ac-to-dc bidirectional converters are expected to be used in future vehicle-to-grid (V2G) applications. Because a battery's operating voltage range depends on the battery's state of charge, achieving high efficiency across the entire operating voltage range of the battery is a major design challenge in bidirectional converter designs.
FIG. 1(a) is a block diagram showing the power stage and the control of a dual-active-bridge (DAB) converter, which is a topology widely used in bidirectional isolated converters. FIG. 1(b) shows timing waveforms of bridge voltages VAB and VCD for the DAB converter of FIG. 1(a). Also illustrated in FIG. 1(b) is phase-shift Φ between bridge voltages VAB and VCD, which is used in the DAB converter to control power flow. When phase-shift Φ is positive, power flows from source V1 to source V2 (i.e., source V1 delivers power while source V2 receives power). When phase-shift Φ is negative, power flows in the reverse direction, so that source V2 becomes the power source and source V1 becomes the output device or the load. As the output side of a converter typically requires regulation, FIG. 1(a) shows a bidirectional DAB converter that includes two output or load feedback control loops for load control. At any given time, only one of these two output-control loops is active: (a) when phase-shift Φ is positive, the loop regulating source V2 is active; and (b) when phase-shift Φ is negative, the loop regulating source V1 is active. Depending on the characteristics or nature of sources V1 and V2 of FIG. 1(a), the output or load control loops may be set up for regulating respective voltages, currents or power.
Descriptions of various aspects of DAB converter performance optimization can be found in the technical literature, with most papers focused on efficiency improvements through power-stage refinements and advanced control techniques, such as duty-ratio modulation of the switches in the individual bridges. However, transformer saturation—which is an issue of paramount importance for reliable operation of isolated bidirectional converters—is not sufficiently addressed in the DAB converter literature, even though isolated bidirectional converters are more susceptible to transformer saturation than their unidirectional counterparts. Transformer saturation arises in bidirectional converters because the primary and secondary sides of the transformer are both connected to voltage sources. There are two major causes of volt-second imbalance across a transformer winding. The first is a duty cycle difference between the positive and negative periods of bridge voltage VAB (i.e., DPP≠DPN), as well as a duty cycle difference between the positive and negative periods of bridge voltage VCD (i.e., DSP≠DSN), or both (see, FIG. 1(b)). Such duty cycle differences may be caused by a mismatch in the timing of the drive signals of the switches in each bridge. The second cause of volt-second imbalance is a difference in positive and negative winding voltage of winding voltages (i.e., VABP≠VABN, VCDP≠VCDN, or both). The difference in winding voltage levels may be caused by unequal voltage drops across semiconductor switches. A volt-second imbalance creates an imbalance in the negative and positive flux changes in the magnetic core of the transformer, which eventually results in transformer saturation.
Generally, the passive and active approaches for eliminating transformer saturation that are applicable to unidirectional isolated full-bridge converters are also applicable to bidirectional converters. Passive approaches include (a) designing the transformer with the goals of a low peak flux density and a large core gap, so as to absorb the anticipated worst-case flux imbalance without saturating the core; and (b) adding blocking capacitors in series with the primary winding and/or the secondary winding of the transformer to eliminate DC currents. However, these passive approaches are not desirable because (a) designing a transformer too conservatively leads to a larger transformer core, or increases the peak value of the magnetizing current, thus increasing conduction and switching losses; and (b) adding blocking capacitors requires additional components, thereby increasing both the size and the cost of the converter.
For unidirectional isolated converters, many active approaches have been introduced that are based on sensing transformer currents and using the sensed signals to modify durations of the driving signals for the switches, thereby maintaining flux balance. Some examples include: (a) the article, “A Flux Balancer for Phase-Shift ZVS Dc-Dc Converters under Transient Conditions,” by J. Claassens and I. Hofsajer, published in Proc. of IEEE Applied Power Electronics Conference (APEC), 2006, pp. 523-527; (b) U.S. Pat. No. 3,870,943, entitled “Converter Circuit with Correction Circuit to Maintain Signal Symmetry in the Switching Devices,” by H. Weischedel and G. Westerman, issued Mar. 11, 1975; (c) U.S. Pat. No. 4,150,424, entitled “Dynamic Current Balancing for Power Converters,” by P. Nuechterlein, issued Apr. 17, 1979; (d) U.S. Patent Application Publication 2013/0088895, entitled “Full Bridge Converter,” by Z. Ye and S. Xu, published Apr. 11, 2013; and (e) the article, entitled “Zeroing Transformer's DC Current in Resonant Converters with No Series Capacitors,” by A. Gertsman, and S. Ben-Yaakov, published in Proc. of IEEE Energy Conversion Congress and Exposition (ECCE), 2010, pp. 4028-4034.
Examples of methods of preventing transformer saturation in DAB converters include: (a) the article “‘Magnetic Ear’—Based Balancing of Magnetic Flux in High Power Medium Frequency Dual Active-Bridge Converter Transformer Cores” (“Ortiz I”), by G. Ortiz, J. Mühlethaler, and J. W. Kolar, published in Proc. of IEEE 8th International Conference on Power Electronics, ECCE Asia Conference, 2011, pp. 1307-1314.; (b) the article “Flux Balancing of Isolation Transformers and Application of ‘The Magnetic Ear’ for Closed-Loop Volt-Second Compensation” (“Ortiz II”), by G. Ortiz, L. Fassler, J. W. Kolar, and, O. Apeldoorn, published in IEEE Transactions on Power Electronics, May-June 2013, pp. 1307-1314; and (c) the article “Preventing Transformer Saturation in Bi-Directional Dual Active Bridge Buck-Boost DC/DC Converters” (“Han”), by S. Han, I. Munuswamy, and D. Divan, published in Proc. of IEEE Energy Conversion Congress and Exposition (ECCE), 2010, pp. 1450-1455.
Ortiz I and Ortiz II each disclose a flux-density transducer which measures flux density in the core of the transformer and which eliminates its dc component by an active flux-balancing control loop. Ortiz I and Ortiz II also review both existing direct and indirect sensing and measurement methods for the magnetic flux in the core of a transformer, and passive and active methods for preventing core saturation.
Han discloses a method for preventing transformer saturation in a DAB-Buck-Boost (DAB3) converter which uses an active flux-balancing method. Under that flux-balancing method, the DC components of the primary and secondary currents of the transformer are made substantially zero by sensing average primary and secondary currents and injecting signals proportional to their values into the sensed filter inductor current. Using a peak-current control approach, the inductor current is used to adjust and to maintain a flux-balance between the primary and secondary windings.